This invention relates to a method and apparatus for the manufacture of an optical fiber preform having incorporated therein a predetermined and enhanced amount of rare earth dopant material, and particularly, wherein the rare earth dopant material is incorporated at a comparatively high concentration and with a cross-sectional geometry of the preform designed to promote good mode scrambling.
Optical fibers are essentially ultra thin light conduits. Light is pumped into one end, propagates forward within and through the fiber, whether bent or straight, and ultimately emerges at the other end. By pumping light into the fiber in a predefined pattern, huge amounts of information can be communicated over large bandwidths over long geographic distances almost instantaneously (i.e., at the speed of light). Thin, fast, and robust, the utility of optical fibers is beyond question.
While the variety, forms, and complexity of fiberoptic configurations continue to evolve, the central underlying structure found in virtually all optical fibers is a light transmitting core surrounded by a cladding layer. The indices of refraction of the core and the cladding are adjusted during manufacture to provide the cladding with an index of refraction that is less than that of the core. When light is pumped into the fiber core, it encounters the refractive index differential at the core/cladding interface and in an optical phenomenon, also referred to as xe2x80x9ccontinuous internal reflectionxe2x80x9d, is xe2x80x9cbentxe2x80x9d back with little loss into the core, where it continues to propagate down the optical fiber.
In manufacture, an optical fiber is typically drawn from an optical fiber preform that essentially has the same cross-sectional geometrical arrangement of core and cladding components as that of the final optical fiber, but a diameter several orders of magnitude greater than that of the fiber. One end of the preform is heated in a furnace to a soft pliable plastic consistency, then drawn lengthwise into a fiber having the desired fiber core/cladding dimension. In the art of fiber preform manufacture for transmission fibers, as opposed to the manufacture of active fibers, i.e., fibers with rare earth doped cores in single mode or double clad fibers, techniques have been developed for high speed manufacture to reduce costs while providing high quality fiber using chemical deposition processes where constituents in their vapor phase are supplied to a horizontally rotated refractory tube to form one or more inner glass layers on the inside surfaces of the tube. Examples are the MacChesney at al. U.S. Pat. No. 4,909,816 and its companion U.S. Pat. Nos. 4,217,027 and 4,334,903, disclosing what is referred to as the modified chemical vapor deposition (MCVD) process, named as such to distinguish it from general semiconductor type CVD processes as well as from prior CVD process employed for the manufacture of glass preforms. These patents discusses the so called xe2x80x9csootxe2x80x9d or outside vapor deposition (OVD) process, disclosed in U.S. Pat Nos. 3,775,075 and 3,826,560, which process is enhanced by the use of the MCVD process. Patent ""816 relates to the establishment of a more prominate homogeneous reaction where the reaction product from the vapor phase forms glass precursor particulates within the gas stream within the ambient of the refractory tube which particulates are then subsequently deposited downstream of the heat zone or source on the inner surface of the tube. The deposited particulates are then consolidated into a transparent glass layer on the tube surface by the passing heat zone. This is distinguished from previously employed CVD processes for glass preforms where a heterogeneous process is explained to occur with glass particulates initially formed on the inner surface of the refractory tube forming either a soot layer that is subsequently sintered to form a glass layer or directly forming a glass layer resulting in deposited formation of monolithic glass, as opposed to glass particulates initially formed within the ambient of the glass refractory tube. The homogeneous reaction of the MCVD process is accomplished, in a significant manner, by increasing the temperature of the reaction zone via the hot moving zone. The advantage of the MCVD approach over the OVD process is eliminating hydrogen bearing components, water vapor and other contaminants from the deposited glass layer. The MCVD process is explained briefly in Andrejco et al. U.S. Pat. No. 4,257,797 and is explained in detail in the book entitled xe2x80x9cOptical Fiber Communicationsxe2x80x9d, Vol. 1, Fiber Fabrication, edited by Tingye Li, 1985 (Academic Press, Inc.), in particular, at pages 1-64, which is incorporated herein by reference.
From the point of view of patent ""816, higher productivity of glass preforms for large scale fiber production, via subsequent fiber drawing, can be achieved by providing a continuous, unbroken processing procedure which includes increasing the reaction temperature for glass layer formation, increasing the rate of tube rotation, sintering the deposited glass layer, minimizing the effects of hydration contamination from the deposited glass layer while rotating and collapsing the tube to form the preform. While the high speed process approach may be highly applicable to manufacture transmission fiber, it is not a preferred approach to the manufacture of active fiber, particularly where high levels of a rare earth dopant or codopants are desired for incorporation in the deposited layer or layers on the inner surface of the refractory tube. Active optical fibers are employed as fiber gain media for purpose of signal amplification of fiber laser applications and are comprised of a single mode fiber or a double clad fiber with a core composition doped with 4f rare earth elements (i.e., the lanthanide series of element, atomic numbers 57-71), e.g. erbium or ytterbium or co-doped with erbium and ytterbium. By selective use of particular concentrations and/or mixes of rare earth dopants, the spectral absorptivity of the core to certain wavelength ranges of light can be defined to desired specifications. An appropriately tuned core, surrounded with an appropriate cladding configuration, can provide, in combination with an appropriate pump source, the basis for light lasing and/or light amplifying functionality. In consideration, for example, of the need for signal amplification in fiberoptic telecommunication projects, optical fibers capable of such light intensifying functionality are desirable. Unfortunately, rare earth doping is not easy performed, particularly at high levels of concentrations in the core.
Various methods and variation have been developed for fabricating rare earth doped optical fiber preforms. Some examples of these methods are disclosed in U.S. Pat. No. 4,501,602, issued to Miller et al. on Feb. 26, 1985; U.S. Pat. No. 4,616,901, issued to MacChesney et al. on Oct. 14, 1986; U.S. Pat. No. 5,236,481, issued to Berkey on Aug. 17, 1993; U.S. Pat. No. 5,609, 665, issued to Bruce et al. on Mar. 11, 1997; U.S. Pat. No. 4,501,602, issued to Miller et al. on Feb. 26, 1985; U.S. Pat. No. 4,826,288, issued to Mansfield et al. on May 2, 1989. Regardless, under current practice, it is very difficult to incorporate high concentrations of rare earth dopants at limited total doping levels, particularly, in the case of the popular rare earth element neodymium (Nd).
Part of the problem is the need in one of the most commonly-practiced preform manufacturing methodologies, i.e., MCVD, to generate and deposit as layer a vapor laden with rare earth dopant. Under current practice, it is difficult to generate anything other than relatively low vapor pressures, resulting ultimately in the incorporation of correspondingly low concentrations of rare earth dopant.
Without the ability to attain a high rare earth dopant concentration, one cannot produce an optical fiber with a low numerical aperture, a low core attenuation, and high pumping power absorption, all of which are desired criteria in the design of fiberoptic lasers and amplifiers.
Additionally, in regard specifically to fiberoptic lasers, even if a suitable fiber optic preform is made, the lasing efficiency of a fiber drawn therefrom may still suffer in other respects. The performance of fiber lasers, as in any active or nonlinear waveguide, is related intimately to the efficiency with which pump radiation can be absorbed by the active material in the fiber core. In the earliest fiber lasers, an appreciable amount of the radiant energy pumped into the fiber would not pass into the core, and, thus, did not contribute to the core""s lasing effect. In response, various cross-sectional fiberoptic geometries, in particular, pertaining to the cross-sectional geometry of the inner cladding of a double clad fiber, were successfully developed that are capable of effecting patterns of internal reflection having a greater frequency of core interactions with light propagating along the inner cladding and crisscrossing and being absorbed in the doped core. See, for example, U.S. Pat. No. 4,815,079, to Snitzer et al. issued Mar. 21, 1989; and U.S. Pat. No. 5,533,163, to M. H. Muendel issued Jul. 2, 1996. However, designing a fiber on the basis of such learning requires additional manufacturing steps in the preform formation. Any improvement that would reduce the burden of these additional steps with the enhancement of light scattering in the inner cladding for enhancing absorption in the fiber core would be desirable.
According to this invention, a methodology and apparatus for the manufacture of an optical fiber preform is provided having incorporated therein a comparatively high concentration of rare earth dopant material, and which thus can be drawn and processed into an optical fiber having low numerical aperture, low core attenuation, and high pumping power absorption. The high concentrations of rare earth dopant material are attained in the practice through the employment of either what we refer to as the xe2x80x9chybrid vapor processingxe2x80x9d (HVP) method or a xe2x80x9chybrid liquid processingxe2x80x9d (HLP) method, each capable of being practiced in combination or independently of one another. The methods of application and the apparatus to practice the methods herein are applicable to formation of glass performs for most types of fiber geometry, including single mode fiber and double clad fiber, having high optical homogeneity. The HVP method involves the vaporization of a solid state form of a rare earth chloride by the exposure thereof to a sufficiently elevated temperature, contemporaneously with the transport of the resultant rare earth chloride laden vapor into an oxidation reaction zone within the bore of a hollow refractory tube on a flowing stream of essentially unreactive inert gas, such as helium. A vapor of glass forming material, e.g., SiCl2, is introduced contemporaneously into the reaction zone. By regulating the temperature of the reaction zone, one or more layers of glass, whether deposited in the form of a soot layer, or as a monolithic, sintered glass layer can be directly deposited as in the case taught in U.S. Pat. No. 4,909,816 and some earlier patents. The soot layer is deposited on the inner surface of the bore of the refractory tube by oxidation of constituents comprising the rare earth chloride laden vapor and the vapor of glass forming material. The hollow tube is thereafter collapsed to form the optical fiber preform.
As used herein, the term, xe2x80x9csoot layerxe2x80x9d, is a deposited layer having a large amount of porosity and is not fully sintered to form a glass or amorphous layer and, therefore, lacking any optical transparency, optical properties and homogeneity as found in monolithic glass layer formed after a high temperature sintering step.
An important feature of the HVP method is the employment of a rare earth dopant deliver system that provides for rare earth laden vapor from its solid state form in advance of mixing with oxygen or oxides of glass forming materials introduced in the vapor phase deposition (VPD) process. In other prior art methods, such as disclosed in U.S. Pat. No. 4,909,816 and its companion patents, the rare earth vapor comes in contact almost immediately with oxygen or oxides thereof. We have found that this has a definite and profound effect on the uniformity of constituents in the deposited soot layer deposited on the inside of the refractory tube. There is not a uniform incorporation of the rare earth component on a continuous, repeatable basis let alone the incorporation of intermediates that function as homogenizers. The HVP method of this invention provides for uniform, repeatable incorporation of rare earth and/or intermediate components with comparatively high levels of concentration through the employment of the novel delivery system of this invention forming a layer of high optical homogeneity. By high optical homogeneity, we mean that the resultant layer of deposited and sintered monolithic glass that has irregularities in the deposited glass material less than about 2 xcexcm in width or less. Anything larger than this is referred as having heterogeneity and considered unacceptable in that the glass material has not been fully reacted and undergone a sufficient transformation into an amorphous, monolithic glass layer with uniform mixed glass components including intermediates and rare earth dopant uniformity.
The HLP method involves the method of depositing a glass layer or layers containing a first amount of rare earth dopant material on the inner surface of a refractory tube forming a soot layer or layers on the internal surface. The layer or layers are deposited at a temperature to provide a soot consistency with multiple pores without transformation into a continuous monolithic glass layer. This step may be carried out employing the HVP method or employing a standard VPD process of the prior art. In either case, the soot-deposited refractory tube is then removed from the preform lath and impregnated with a dopant solution formulated with a second amount of rare earth dopant material. The tube is then return to the preform lath, heated to sinter the doped impregnated layer or layers and thereafter collapsed, resulting in an optical fiber preform with a final amount of rare earth dopant material that includes substantially both the first and second amounts of rare earth dopant material.
An optical fiber preform made according to the HVP or the HLP method can be employed with its geometry as formed, or the preform geometry may be modified before the preform is drawn into fiber to introduce deviations in the optical properties of the glass preform, e.g., a light scattering mechanism. Mechanical grinding or a chemical process may be employed to form a simple flat or concave surface on at least one longitudinal surface of the preform. These are quite suitable for changing the preform geometry prior to the fiber drawing process and the formation of an outer cladding layer as in the case of drawing a double clad preform or a sleeve as may be the case in drawing a single mode fiber. More than one flat can be applied to the preform such as on opposed longitudinal surfaces of the glass preform.
In light of the above, it is principal object of this invention is to provide improved methodology and apparatus for the manufacture of an optical fiber preform having a comparatively high rare earth dopant concentration, particularly, wherein the limited total doping concentration of the glass fiber preform is at a level sufficient to effect a low numerical aperture in an optical fiber prepared therefrom.
It is another object of this invention to provide a method of manufacturing of an optical fiber preform wherein enhanced, higher levels of incorporation of a rare earth dopant or dopants is achieved by the vaporization of a solid state form source of a rare earth material in close proximity to the region of deposition of the glass forming material on the inner surface of a rotated refractory tube. More than one such rare earth source may be employed respectively providing different vapor laden rare earth dopants to the reaction zone within the refractory tube.
It is another object of this invention to provide a method of manufacturing an optical fiber preform wherein the incorporation therein of rare earth dopant material is effected by performing a soot layer deposition rare earth vapor doping followed by rare earth solution doping.
It is a further object of this invention to provide a method of manufacturing an optical fiber preform, the method characterized by a vapor phase deposition process having means employed therein for reducing the incidence of premature particle-creating oxidation reactions, and thus, ultimately better suited for formation of uniform layer or layers of monolithic glass.
It is a still further object of this invention to provide an method of manufacturing an optical fiber preform, wherein said method is comparatively less susceptible to water contamination, which is often traceable to the inherent moisture sensitivity of halogen-based dopant materials (e.g., aluminum chlorides, rare earth chlorides, etc.) commonly used in preform manufacture.
It is an additional object of this invention to provide a method of manufacturing an optical fiber preform, wherein said method employs a rare earth cyclopentadienide (CP3) and/or derivatives as dopant material.
It is still another object of this invention to provide a method of manufacturing an optical 5 fiber preform, wherein, among other steps, a solid state form of a rare earth halogen, i.e., a rare earth chloride, is vaporized by exposure to a sufficiently elevated temperature, and transported on a flowing stream of essentially unreactive inert gas (e.g., helium) to an oxidation reaction zone, contemporaneously with the introduction thereto of vaporous glass forming material.
It is another object of this invention to provide a method of modifying an optical fiber preform by introducing deviations in the preform geometry to change its optical characteristics.
The foregoing and other features and advantages of the invention will be apparent from the following, more particular description of several preferred embodiments of the invention, as illustrated in the accompanying drawings.